Magnetic resonance imaging (MRI) is an imaging technology involving biomagnetics and nuclear spin, which has developed rapidly together with computer technology, electronic circuit technology and superconductor technology. It uses a magnetic field and radio frequency (RF) pulses to make hydrogen nuclei (i.e. H+) which are precessing in human tissue experience nutation and generate RF signals, which are processed by computer to form an image. By placing an object in the magnetic field, using suitable electromagnetic waves to irradiate it so that it resonates, and then analysing the electromagnetic waves it releases, the positions and types of the atomic nuclei which make up the object can be discovered, and on this basis it is possible to draw a precise three-dimensional image of the interior of the object. For instance, a human brain can be scanned using MRI to obtain an animated image of contiguous slices, from the top of the head all the way to the base.
Compared to other medical imaging products such as X-ray products and CT products, MRI systems can provide high-contrast images of different soft tissues. Excellent image quality is the foundation of medical imaging products, so it is necessary to carry out image quality assessment in the system development stage as well as in the product delivery stage, in order to guarantee quality. In order to establish an image quality assessment system, many standards have been formulated, the most important of which is IEC62464-1(2007), “Magnetic resonance equipment for medical imaging—Part 1: Determination of essential image quality parameters”. This standard was adopted by the Chinese national standard YY/T0482-2010 in July 2012. This national standard is a necessary condition for successful registration in China of all MRI products.
At present, in order to assess image quality, it is necessary to use a designed phantom filled with an aqueous agent or an oily agent. Phantoms are already widely used for assessing the image quality of medical imaging equipment, and are capable of meeting the requirements of many image quality indices by being designed to have different structures and/or to be filled with different solvents. The results of assessment are highly accurate, and reproducible.
A test of image quality for an MRI system includes the following six indices: signal-to-noise ratio (SNR), homogeneity, layer thickness, geometric deformity, resolution and ghost images. In the prior art, tests for layer thickness and resolution must be conducted separately using two different types of phantom; furthermore, no single multi-functional phantom for all six of the above indices currently exists. To test each image quality index, it is necessary to image different phantoms separately, but the use of different phantoms to test different image quality indices will give rise to the following technical problems:
1) In order to test different image quality indices, the operator must continually change different phantoms, which is a time-consuming and laborious process.
2) A maximum of six phantoms are needed for all six of the image quality indices, therefore human labor and material resources are needed to store and maintain each phantom.